The present invention relates to multi-layer structures containing liquid-crystal layers. In particular, the present invention relates to a process for applying an absorption layer of an optically addressable spatial light modulator (OASLM).
OASLMs are used for various areas of optical telecommunications. An example that may be mentioned is that of level balancing in WDM multi-channel systems, as described in W. P. Beard et al., xe2x80x9cAC Liquid Crystal Light Valvexe2x80x9d, Appl. Phs. Lett., Vol. 22, No. 3, 90-92 (1973).
Other applications relate to the local attenuation of strong light sources as glare protection, as well as applications for preventing halation of the image in video cameras, video spectacles, pilot spectacles etc.; see W. P. Bleha, L. P. Lipton, E. Winere, Opt, Eng., Vol 17, No.4, Appl. Phys. Lett, V, Opt. Eng., V, 371-384 (1978).
Particularly important for the optical parallel processing of images is the conversion of optical, incoherent images into the coherent beam path of the optical correlator. Used for this purpose are OASLMs in which the writing and read-out beam paths are optically separated from each other. In the event that the read-out light overlaps spectrally with the sensitivity curve of the photoconductor, it must be completely separated from the writing beam path, because otherwise there are great losses in terms of the sensitivity, resolution and grey-scale range of the OASLM due to this parasitic illumination.
Only in relatively rare cases is it possible to optically separate the writing and read-out beam paths solely through the spectral composition of writing and read-out light, by employing photoconductors which absorb exclusively in the blue light range, while red or infrared light is used for the read-out.
None of the conventional reflecting layers made of metal films or dielectric films have a sufficiently low transmission in order to separate the two beam paths and to prevent the penetration of the read-out light into the photoconductor.
An appreciable improvement in light separation can be achieved by additional absorbing layers between the photoconductor and the reflecting layer of the read-out beam path. A semiconductor layer can be used as the absorbing layer if the band gap is smaller than that of the photoconductor.
Known from W. P. Beard et al., xe2x80x9cAC Liquid Crystal Light Valvexe2x80x9d, Appl. Phs. Lett., Vol. 22, No. 3, 90-92 (1973), W. P. Bleha, L. P. Lipton, E. Winere, Opt. Eng., Vol. 17, No.4, Appl. Phys. Lett, V, Opt. Eng., V, 371-384 (1978) and U. Efron, J. Grinberg, P. O. Braatz, M. J. Little, P. G. Reif and R. N. Schwarts, xe2x80x9cThe silicon liquid-crystal light valvexe2x80x9d, J. Appl. Phys., Vol. 57, No. 4, 1356-1368 (1985) is the use of a cadmium-telluride layer CdTe as a light-blocking layer. Therein described is a cadmium-telluride layer CdTe of 2xcexc thickness, a surface resistance of 1011 xcexa9cm and an absorption coefficient not less than 105 cmxe2x88x921 at 525 nm in structures which used cadmium-sulfide CdS as photoconductor. The same light-blocking layer was also employed to improve the properties of light valves on the basis of liquid crystals with amorphous silicon (xcex1-Si) as photoconductor. The CdTe layer was applied between the dielectric mirror (based on titanium oxide) and a special adhesive layer (to improve the adhesion of the CdTe to the photoconductor). The adhesive layer included 4 films:
1) an SiO2 film applied in an argon atmosphere,
2) an SiO2 film applied in an argon-oxygen atmosphere,
3) a CdTe film likewise applied in an argon-oxygen atmosphere, and
4) a CdTe film applied once again in an argon atmosphere.
Amorphous carbon xcex1-C and amorphous hydrogenated carbon xcex1-C:H as such are known as thin films with high light absorption. These materials can be applied by sputtering or vaporization of carbons or by chemical vapor deposition (CVD), with hydrocarbons as precursor; see A. V. Balakov, E. A. Konshina, xe2x80x9cMethods of production and properties of diamond-like carbon filmsxe2x80x9d, Sov. J. Opt. Technol.xe2x80x9d, Vol. 49, No. 9, 591-599(1982) and A. Bubenzer, B. Dischler, G. Brandt, P. Koidl, xe2x80x9cOptical properties of hydrogenated hard carbon thin filmsxe2x80x9d, Thin Solid Films, Vol. 91, 81-87 (1982). Used for this purpose are plasma discharges in the radio-frequency range and magneton gas discharge; see A. Bubenzer, B. Dischler, A. Nyaiesh, xe2x80x9cRF-plasma deposited amorphous hydrogenated hard carbon thin films. Preparation, properties and applicationsxe2x80x9d, J. Appl. Phys., Vol. 54, 4590-454595 (1983), and D. R. McKenzie, R. C. McPhedran, N. Savvides, xe2x80x9cAnalysis of films prepared by plasma polymerization of acetylene in a d. c. magnetronxe2x80x9d, Thin Solid Films, Vol. 108, 247-256 (1983).
It is also possible to employ direct ion-beam and dual-beam deposition, as well as laser erosion from the carbon target. The optical constants of amorphous carbon layers and amorphous, hydrogenated carbon layers depend characteristically on the deposition and on the parameters of the condensation process; see F. W. Smith, xe2x80x9cOptical constants of a hydrogenated amorphous carbon filmxe2x80x9d, J. Appl. Phys., Vol. 55, 764-771 (1984).
The described carbon layers are used as reflecting layers or as protective layers in IR optics. Optically visible carbon layers with an absorption in the visible spectral range are employed for selective solar applications and as contrast-intensifying layers for crystal displays.
A carbon-containing film of very complex structure of silicon, germanium and carbon is known from K. Takizawa, T. Fuijii, M. Kawakita, H. Kikuchi, H. Fuijkake, M. Yokozawa, A. Murata and K. Kishi, xe2x80x9cSpatial light modulators for projection displaysxe2x80x9d, Applied Optics, Vol. 36, No. 23, 5732-5747 (1997). The layers described therein had a great thickness of 3.5 xcexcm and more, in order to provide the required absorption and the necessary resistance in an OASLM based on thick (250 xcexcm) BSO photoconductor layers and polymer-dispersed nematic liquid crystals.
For xcex1-Si:H and xcex1-Si:C:H photoconducting layers, these blocking layers are too thick to be able to optimize the basic parameters of the OASLM. A further disadvantage is that these complex compositions of Si, Ge and C are too expensive in manufacture.
The present invention provides a process for applying a light-blocking layer between a photoconducting layer and a mirror when manufacturing an optically addressable spatial light modulator OASLM using the CVD process. The light-blocking layer (4) and the photoconducting layer (3) are applied in a shared process step in which both the thickness and composition of the photoconducting layer (3) to be applied to the transparent electrode (2), as well as the thickness and composition of the light-blocking layer (4) to be applied to the photoconducting layer (3) are determined by a time-related change of the variation of the gas composition during the deposition process. The silanes are continuously replaced by hydrocarbons in the gas atmosphere of the CVD reactor, with the result that a layer sequence composed of photoconducting layer (3) and light-blocking layer (4) is produced, having the desired specific resistances.
The process of the present invention relates to the decoupling of the writing and read-out beam paths of an optically addressable spatial light modulator (OASLM) in reflection mode by applying a light-blocking layer between the writing and read-out beam paths. In order to determine and optimize the properties of the light valve, it should be possible during the process to selectively adjust, within narrow limits, the specific resistances of the layers and therefore the conductivity of the layer sequence of transparent electrode ITO, photoconducting layer and light-blocking layer. At the same time, one should be able to implement the process as simply and cost-effectively as possible.
Transparent electrode ITO 2 is applied onto the class or quartz-glass substrate 1 according to already known process steps of the CVD process in a CVD reactor. According to the present invention, photoconducting layer 3 and light-blocking layer 4 are deposited in one process step. First, in a silane atmosphere, a photoconducting layer 3 of amorphous hydrogenated silicon is deposited on glass or quartz-glass substrate 1, which is coated with transparent electrode ITO 2. Through continuous replacement of the silanes by hydrocarbons in the gas atmosphere of the CVD reactor, a light-blocking layer 4 is deposited, with direct-current discharges, on photoconducting layer 3. Depending on the (gas composition during the deposition process, a light-blocking layer 4 of amorphous carbon xcex1-C or amorphous hydrogenated carbon xcex1-C:H is produced. Both the thickness of photoconducting layer 3 and the thickness of light-blocking layer 4 are adjusted by the time-related continuous change of the variation of the gas composition during the deposition process, so that a layer sequence is produced having the desired specific resistances. This means that the specific resistance of the layer sequence composed of transparent electrode/ITO 2, photoconducting layer 3 and light-blocking layer 4 is a function both of the respective gas composition during the deposition process and of the length of the deposition process itself. Mirror 5, orienting layer 6 and liquid crystal 7 are applied according to already known process steps.
Light-blocking layer 4, made of amorphous carbon or of amorphous hydrogenated carbon and acting as an absorption layer, is produced by continuously feeding acetylene into the argon (as atmosphere of the CVD reactor and, with direct-current discharges, a light-blocking layer 4 of pure carbon is deposited on photoconducting layer 3. If a layer of acetylene-argon is deposited in a direct-current discharge according to the CVD process, the specific resistance of light-blocking layer 4 is of the order of magnitude of 108 ohm cm. The greater the proportion of acetylene in the acetylene-argon as atmosphere of the CVD reactor, the higher becomes the specific resistance of light-blocking layer 4. Light-blocking carbon layers xcex1-C:H deposited from a pure acetylene gas atmosphere have a specific resistance of approximately 1012 ohm cm.